In recent years, liquid crystal display devices have been broadly used as monitors for portable notebook-size personal computers and the like, or as display units for liquid crystal television, video integrated liquid crystal television and the like, and further in other various fields. The liquid crystal display device basically comprises a backlight and a liquid crystal display element. As the backlight, an edge light system has been frequently used from a viewpoint of miniaturization of liquid crystal display device. The backlight has been heretofore broadly used in which at least one end face of a rectangular plate-shaped light guide is used as a light incident face, a linear or rod-like primary light source such as a straight tube type florescence lamp is disposed along the light incident face, and the light emitted from the primary light source is introduced into the light guide from the light incident face of the light guide, and emitted from a light emitting face that is one of two major surfaces of the light guide.
Moreover, in recent years, there have been demands for miniaturization as well as reduction of power consumption with respect to liquid crystal display devices having comparatively small screen dimensions, such as indicators for portable electronic apparatuses including a cellular phone, portable game machine and the like, or various electric or electronic apparatuses. Then, a light emitting diode (LED) which is a spot light source is used as the primary light source of the backlight for the reduction of the power consumption. As the backlight in which the LED is used as the primary light source, for example, as described in JP(A)-7-270624, a plurality of LEDs are one-dimensionally arranged along the light incident face of the light guide in order to exert a function similar to that of a backlight using a linear light source. When the primary light source with one-dimensional arrangement of a plurality of LEDs is used, a required light quantity, and uniformity of luminance distribution over the whole screen can be obtained.
Furthermore, in JP(B)-7-27137, a method has been proposed in which a light guide including a rough surface formed on a light emitting face, comprising micro concave/convex portions as a light emission control function structure, is used. A prism sheet having a surface on which a large number of elongated prisms are arranged/formed is disposed on the light emitting face of the light guide in such a manner that the elongated prism formed surface is disposed on a light guide side (i.e., the elongated prism formed surface constitutes a light incident surface), so that the power consumption of the backlight is suppressed, and distribution of the emitted light is narrowed in order to prevent luminance from being sacrificed to the utmost.
Additionally, in recent years, thinning of the light guide and further the prism sheet which is a light deflector has been advanced even in the backlight based on demands for lightening and thinning of various apparatuses such as a liquid crystal display device.
In this case, even when the light incident surface of the prism sheet is used as the elongated prism formed surface as described above, adhesion or sticking sometimes non-uniformly occurs between the light guide and prism sheet formed of a dielectric material such as a synthetic resin, by attractive forces therebetween, such as an electrostatic attractive force. There has been a problem that an apparent defect of the planar light source device is made, and a quality level thereof drops, when the sticking occurs.
In JP(A)-10-48429, it has been proposed that the light emitting face of the light guide be formed into a mat-finished surface having an arithmetic average roughness (Ra) of 0.02 to 0.25 μm in order to prevent the occurrence of the sticking between the light guide and the prism sheet in the planar light source device. However, the occurrence of the sticking can be prevented to a certain degree on the mat-finished surface, but this has not been sufficiently satisfactory yet. The mat-finished surface of the light guide only has a purpose of preventing the occurrence of the sticking, and a mat structure which is a light emission control function structure has not been considered.